Materials which are thermally conductive yet electrically insulative are needed for applications in electrical circuitry, such as for potting or bonding electrical components to cooling structures or systems. A thermally conductive path may be required, for example, between a semiconductor chip and heat sink; but the path must be electrically nonconductive to prevent shorts. Materials used for this purpose have comprised epoxy resin and fillers such as boron nitride, silver, alumina, and beryllia.
The use of epoxy resin pastes and films having diamond particles is disclosed in European Patent Application Publication Number 0 379 773 A1, filed Apr. 7, 1989. An epoxy resin paste containing 80% by weight diamond filler was compared favorably in terms of thermal conductivity and electrical resistivity to an epoxy containing a silver filler. The resin, 60% by volume of diamond particles, was Tra Con FS 280, sold by TRA-CON, Inc. of Medford, Mass.
Until the present invention, the full benefits of using diamond particles for improved thermal conductivity has not been realized or suggested. It is observed that the thermal conductivity value for a polymer alone (e.g., epoxy with no filler) is about 0.2 (W/m.C); the value for diamond alone is 26 (W/m.C). The EP publication teaches that epoxy containing "81% (by weight) diamond filler" provides an average thermal transfer value of 2.8 (W/m.C) at most. This value is far short of the 26 W/m.C of diamond. Although it was suggested in the EP publication that small diamond particles be used to fill the voids between large particles, the resin-based system disclosed therein did not ostensibly approach the heat transfer performance ideally realizable (26 W/m.C) through the use of neat diamond. This is believed to be due to the low (60%) volume percentage content of diamond. The EP publication does not suggest how to load the final volume fraction, containing about 40% volume of the epoxy resin, with the diamond filler.